Looping a walk animation
Loops are generally programmed "in place", having the feets sliding on the floor, and then a forward motion is applied to the whole armature, to simulate a real walking. If you don't want to follow this scheme, you will have to to set keyframes to have your character "jump" forward every frame your animation "jumps" backward. If the loop works the amount of movemente will be always the same, so the programming will be easy, even with long walks.